1. Field of the Invention
The present invention relates to a four-wheel drive system including torque transmission means having an oil pump and a hydraulic clutch between the front and rear wheels of an automobile.
2. Description of the Prior Art
A four-wheel drive car, which is equipped with an oil pump to be driven by the difference in the rotating speeds of two rotating shafts connected to car front and rear wheels and a clutch for connecting and disconnecting said two rotating shafts in dependence upon the discharge pressure of the oil pump according to the difference in the rotating speeds, is well known in the art, as is disclosed in Japanese Patent Laid-Open No. 252026/1985, for example.
In this four-wheel drive car, the pressure oil discharged from the oil pump is released through a fixed orifice to the atmosphere so that the discharge pressure of the oil pump may be controlled to a level according to the difference in the rotating speeds of the rotating speed at the front and rear sides. Here, in this four-wheel drive car, the difference in the rotating speeds arises between the rotating shafts at the front and rear wheel sides in case either the front or rear wheel or wheels slip or in case the so-called "inner wheel difference" arises when the car turns.
However, the above-specified two cases of establishing the rotating speed difference between the two rotating shafts are absolutely different. In the former case, the transmission torque of the clutch has to be enhanced to shift the driving state from the two-wheel drive type to the four-wheel drive type. In the latter case, on the contrary, it is necessary to reduce the transmission torque of the clutch thereby to avoid the tight corner braking phenomenon which will occur when the car turns. It is, however, very difficult to control the pressure to levels appropriate for the two individual cases. If the throttle opening of the fixed orifice is set to avoid the braking phenomenon, the discharge pressure of the oil pump cannot be boosted when the front or rear wheel or wheels slip. As a result, the clutch slips when in the four-wheel drive to raise a problem that the power loss is increased.